Ultra-high-molecular weight polyethylene (UHMW-PE), standard high-density polyethylene (HDPE) and low-density polyethylene (LDPE) have all been used as polymeric materials for producing different types of molded porous articles. Such articles include filter funnels, immersion filters, filter crucibles, porous sheets, pen tips, marker nibs, aerators, diffusers and light weight molded parts. However, the polyethylene formulations used in these applications are all associated with various disadvantages.
LDPE and standard HDPE, which include polyethylene of molecular weight up to 250,000 g/mol, yield good part strength but their melt behavior results in a narrow processing window with respect to both time and temperature. As result, there is a strong tendency toward reduced porosity and an increased quality inconsistency in the molded product. Furthermore, with LDPE or standard HDPE as the molding powder, the non-uniformity of heating within molds having complex geometric conduits tends to result in non-uniformity in the porosity of the product part.
In contrast to LDPE and standard HDPE, UHMW-PE formulations with an average molecular weight above 2,500,000 g/mol exhibit excellent processing forgiveness. Specifically, it is known in the art that UHMW-PE molding powders are characterized by a wide time and temperature processing window. However, these UHMW-PE formulations are known to result in rather weak molded products. Moreover, regional weak spots tend to be formed when UHMW-PE is used with molds having a complex geometric conduit. To maintain or improve the strength of porous articles made from UHMW-PE, U.S. Pat. No. 4,925,880 to Stein discloses the addition of a polyethylene wax to the UHMW-PE particles. Stein teaches to add the wax in an amount between 5-60% to improve strength and porosity. However, the use of polyethylene wax in this manner restricts the time and temperature processing window and is thus associated with the same disadvantages as using LDPE and standard HDPE.
High molecular weight polyethylenes are valued for properties such as chemical resistance, abrasion resistance, strength, water absorption, energy absorption, heat deflection, and sound-dampening capabilities. Processes for preparing high molecular weight polyethylenes are known in the art. U.S. Pat. No. 4,962,167 to Shiraishi et al. discloses a process for making polyethylene powder by polymerizing ethylene using a solid catalyst component and an organometallic compound. According to the '167 patent the polyethylene powder is reported to have bulk densities from 0.30 g/cc to 0.34 g/cc with particle diameters ranging from 195 to 245 microns.
Another process for making high molecular weight polyethylene is disclosed by U.S. Pat. No. 4,972,035 to Suga et al., whereby polymerization is carried out in the presence of a Ziegler catalyst and the polyethylene is subjected to a high-speed shearing treatment. The morphologies of the particles in Suga et al. are stated to be substantially spherical, with elliptical or cocoon-like shapes.
U.S. Pat. No. 5,587,440 to Ehlers et al. discloses a method for making high molecular weight polyethylene powder with bulk densities ranging from 350 to 460 g/liter using Ziegler type catalysts.
Methods for producing porous articles from high molecular weight polyethylene powders are likewise known. U.S. Pat. No. 3,024,208, to Goethel et al. discloses a process for forming porous bodies by placing the polyethylene powder into containers and heating it under slight pressure. The porous articles produced by the process in Goethel et al. are reported to have densities ranging from 0.33 g/cc to 0.66 g/cc and porosities between 32 and 67%.
Still further processes for making polyethylene articles are noted below.
PCT Application Publication No. WO 85/04365 discloses a sintering process whereby high molecular weight polyethylene powder is pre-compacted under pressure and heat to increase its bulk density. The compacted powders are reported to have bulk densities that are greater than 0.4 g/cc. The bulk density is increased by altering the particles' morphologies (removing the “fine structure”) by passing the powder through a pellet or roll mill. The particle morphology of high molecular weight polyethylene can affect the compaction and sintering behavior of the powder. See, Sangeeta Hambir and J P Jog, Sintering of Ultra High Molecular Weight Polyethylene, Bull. Mater. Sci., v. 23, No. 3 (June 2000).
U.S. Pat. No. 5,977,229 to Barth et al. and U.S. Pat. No. 3,954,927, discloses porous articles, particularly filters, which are sintered from high molecular weight polyethylene.
Copending U.S. patent application Ser. No. 10/640,830 discloses a process for forming a porous article using a molding powder comprising a polyethylene polymer having a molecular weight in the range of about 800,000 to about 3,500,000 as determined by ASTM-D 4020, and a particle size distribution in the range of about 10 microns to about 1,000 microns. The particles are spherical in shape. Commercial examples of resins which can be successfully used in this process are GUR® 4012 and 4022, produced by Ticona LLC (Florence, Ky.). These materials have a powder bulk density in the range of 0.38 to 0.55 gm/cc. Although GUR® 4012 and 4022 can be shaped and sintered to yield articles having good porosity, there is still a need for improved polyethylene resins for preparation of articles having well-controlled porosity and good mechanical strength.